A comprehensive new look at how mothers fill their hours suggests it's time to revise the old saw "A man may work from sun to sun. But a woman's work is never done." The new saw should be, "24/7, there's something to see. Make your own bed, I'm watching TV."

Compared to mothers with kids between ages 5 and 18 at home in 1965, contemporary moms spend on average 11 fewer hours per week in physical activity -- including housework, meal preparation, child care, laundry and exercise. But those same women are logging seven more hours per week in sedentary activity, such as watching TV, surfing the Web or driving.

Moms of children under 5 have shed about 14 hours of physical activity time weekly, but have increased their weekly log of sedentary time by six hours, on average.

The new research, published Monday in the Mayo Clinic Proceedings, looked at data collected in the American Heritage Time Use Study, which has detailed the time Americans engaged in paid and unpaid work from 1965 to 2010. The researchers, led by University of South Carolina public health professor Edward Archer, looked only at how mothers allocated their time outside of hours spent in paid work.

The dramatic reallocation of mothers' time has consequences for the women themselves: The resulting reduction in energy burned up in physical activity averages 177 calories more per week for women with older children at home, and 225 calories per week for those with preschoolers.

And because mothers' patterns of activity, consumption and weight gain profoundly influence those of their kids from gestation onward, this 45-year shift from exertion to inertia has likely played an outsized role in the nation's epidemic of overweight and obesity, the authors say.

“A mother’s physical activity and sedentary behaviors affect the environments to which her progeny are exposed, such as the intrauterine milieu and family social setting,” said Archer. As a result, he added, the change in mothers' habits, in their weight status and in their propensity to diseases is likely being transmitted across generations.

“With each passing generation, mothers have become increasingly physically inactive, sedentary and obese, thereby potentially predisposing children to an increased risk of inactivity, adiposity and chronic noncommunicable diseases. Given that physical activity is an absolute prerequisite for health and wellness, it is not surprising that inactivity is now a leading cause of death and disease in developed nations,” Archer said.

Archer and his coauthors gauged the energy expenditure of mothers by mining the time-use sheets for detailed break-outs of hours spent daily in distinct activities. They assigned different metabolic equivalents -- and thus, calorie expenditure -- to doing laundry and cleaning up after dinner, for instance, than they did to leisure-time physical activity.

In 1965, mothers with older children at home spent, on average, 32 hours in "activity time" -- a mix of exercise for pleasure and active unpaid work such as child care and housework -- and 18 hours in sedentary activities. By 2010, activity time accounted for an average of 21 hours per week for these women, and sedentary time had expanded to 25 hours per week.

Mothers with children under 5 at home went from 44 hours of "activity time" per week in 1965 to an average of 30 hours in 2010. But their sedentary time went from an average of 17 hours per week to 23 hours. Most of those changes came in the 1960s, '70s and '80s, with the allocation of time to sedentary and physical activity having largely stabilized since the 1990s.

Calling physical activity "an absolute prerequisite for health and wellness," Archer noted that it's no surprise that inactivity has become a leading cause of death and disease in developed nations. The expansion of time spent on the couch "may be the greatest public health crisis facing the world today,” Archer and his coauthors wrote.

Of the many powers mothers wield, few are more extraordinary than the power to nudge a child's health prospects in one direction or another simply by having incubated that child in her womb. Research is uncovering more and more instances in which a pregnant woman's own health issues...